The goal of this K01 Award application is to enhance career development of the Candidate, Dr. Eliseev. Under the guidance of Drs. Regis O'Keefe, Matthew Hilton and Paul Brookes, the Candidate will elucidate the link between bioenergetics and osteogenicity of mesenchymal stem cells (MSC) and aging-related changes in this link. These studies will serve as a vehicle for introducing Dr. Eliseev into research areas that are new to him, such as stem cells, bone remodeling and repair, aging, osteoporosis, and mouse genetic models. MSCs use glycolysis for energy production and switch to mitochondrial oxidative phosphorylation during osteogenic differentiation. This bioenergetic switch is disrupted in aging, diabetes and other disorders leading to decreased MSC osteogenicity and osteoporosis. Our long-term research goal is to understand how cell metabolism determines cell fate and how it can be manipulated for the purposes of prevention and therapies. The objective of this proposal is to determine the mechanism underlying changes in MSC bioenergetics during aging and its effect on MSC osteogenicity and bone quality. Activation of the mitochondrial permeability transition (MPT) is a well documented event in cardiovascular and other systems during aging. The role of the MPT in aged bone has not been elucidated. Based on our data and the literature, our central hypothesis is that bioenergetic failure and decreased viability due to the MPT disrupt osteogenic potential of aged MSCs, leading to osteoporosis and delayed fracture healing that can be reversed by inhibition of the MPT. Our specific aims are: (1) determine the mechanism of mitochondrial dysfunction in aged MSCs and its effect on MSC viability and osteogenicity. We hypothesize that the MPT is such a mechanism; (2) elucidate the effect of inhibition of the MPT on osteoporosis during aging. We hypothesize that this will improve bone quality in aged mice; and (3) determine the effect of inhibition of the MPT on fracture healing in aged mice. We hypothesize that this will accelerate fracture healing. To attain our aims we will use MSC biology methods; mouse genetic models of global or MSC-specific loss-of-function of the MPT; and novel pharmacological inhibitors. Our contribution here is expected to be a detailed understanding of how MSC bioenergetics is disrupted in aging and how it can be improved for the purposes of prevention and therapies. This is very significant because it will lead to new strategies for osteoporosis and fracture repair and provide significant benefits for public health. Our research will also advance the field of bone biology and aging by elucidating yet unknown mechanisms connecting MSC bioenergetics and osteogenicity. Our research is innovative because it departs from the status quo and puts impaired bioenergetics in MSCs in the center of pathogenesis of osteoporosis; and tests a novel approach, MPT inhibition, to treat osteoporosis and delayed fracture healing during aging.